The present invention generally relates to engine restart systems and methods and, more specifically, to an emergency power engine restart system and method having separate hot and cold gas turbine wheels and an interconnecting gearbox.
In an aircraft gas turbine engine, an electric starter motor may be used to apply torque to the engine's shaft in order to start the engine. Alternatively, gas turbine engines may be started on the ground by providing a stream of pressurized air from a ground cart to an air turbine starter which can apply torque to the engine's shaft in order to start the engine. As the shaft starts to rotate, air is inducted into the compressor, compressed and then discharged into the combustor. Concurrently, the engine's fuel control system feeds fuel into the combustor in accordance with a pre-programmed fuel schedule in order to precisely maintain the proper fuel/air ratio in the combustor, thereby achieving a light-off condition. After light-off, the starter torque is augmented by torque from the engine's turbine. Before reaching idling speed of the engine, the starter is shut off. This operation constitutes a starting cycle of a gas turbine engine.
Typically, propulsion engines on aircraft will have an accessory gearbox either mounted to the engine or mounted to the airframe. Either way, this gearbox is connected to the shaft of the turbine engine. Typically, there are several “pads” located on this gearbox where several accessory components can attach and be driven by the gear and shafts within the gearbox. The accessories can be pumps, generators, and the like. For example, U.S. Pat. No. 4,372,517 discloses engine accessory pads for mounting a gearbox onto the shaft of a turbine engine.
Engine starters may also be connected to the main engine shaft via accessory gearboxes. U.S. Pat. No. 3,951,008 describes a gearbox used to start a turbine engine. The gearbox may include two inputs and one overrunning clutch. The two inputs are used in sequence to perform the starting process. One starter input is used to drive from zero rpm up to a particular speed, through the clutch. Then, the second starter input acts to slip the overrunning clutch and bring the turbine up to full starting speed. Both inputs are required to bring the engine up to starting speed.
U.S. Pat. No. 5,201,798 describes an assembly which combines the function of both an auxiliary power unit (APU) and an emergency power unit (EPU) into one unit. The unit also includes the primary APU turbine engine. The gearbox utilized in the '798 patent has several output shaft pads to drive pumps and an electrical generator. The '798 patent describes the use of an air turbine starter mounted onto the gearbox via a clutch to permit disengagement once the engine is started. There is also a hot-gas impulse turbine attached to the gearbox via another clutch, which allows for EPU restarts during flight. The '798 system is a single integrated system which may require complex disassembly and maintenance procedures for system servicing.
As can be seen, there is a need for a simple engine restart system which may be attached to the primary engine accessory gearbox via a single drive shaft pad. Moreover, there is a need for an engine restart apparatus and method providing dual turbine inputs for ground starting and in-flight engine restart capability.